The present invention relates generally to electrochemical detection for liquid chromatography, more particularly to flowcell detectors, and more specifically to a new working electrode for use in a flowcell detector.
Arrays of electrodes for use as electrochemical detectors in flowcells are the subject of many ongoing investigations. Originally developed to take advantage of the properties displayed by microelectrodes (i.e. enhanced current densities from non-planar diffusional contributions to the net current, low iR drop characteristics and a decreased dependence on convection) and to generate larger, more easily measured currents, arrays of electrodes have been fabricated from various materials and in different geometric configurations. See, e.g., Weisshaar, D. E., Tallman, D. E., Anderson, J. L., Kel-F - Graphite Composite Electrode as an Electrochemical Detector for Liquid Chromatography and Application to Phenolic Compounds, 53 Anal. Chem. 1809 (1981); Falat, L., Cheng, H. Y., Voltammetric Differentiation of Ascorbic Acid and Dopamine at an Electrochemically Treated Graphite/Epoxy Electrode, 54 Anal. Chem. 2109 (1982); Wang, J., Freiha, B. A., Vitreous Carbon-Based Composite Electrode as an Electrochemical Detector for Liquid Chromatography, 298 J. Chromatogr. 79 (1984); Caudill, W. L., Howell, J. O., Wightman, R. M., 54 Anal. Chem. 2531 (1982); Bond, A. M., Henderson, T. L. E., Thorman, W., Theory and Experimental Characterization of Linear Gold Microelectrodes with Submicrometer Thickness. 90 J. Phys. Chem. 2911 (1986); Thorman, W., van den Bosch, P., Bond, A. M., Voltammetry at Linear Gold and Platinum Microelectrode Arrays Produced by Lithographic Techniques, 57 Anal. Chem. 2764 (1985); Fosdick, L. E., Anderson, J. L., Optimization of Microelectrode Array Geometry in a Rectangular Flow Channel Detector, 58 Anal. Chem. 2481 (1986); Fosdick, L. E., Anderson, J. L., Baginski, T. A., Jaeger, R. C., Amperometric Response of Microlithographically Fabricated Microelectrode Array Flow Sensors in a Thin-Layer Channel, 58 Anal. Chem. 2750 (1986); DeAbreu, M., Purdy, W. C., 32- Gold-Electrode-Array Thin-Layer Flow Cell, 59 Anal. Chem. 204 (1987). Because of the low dead volumes required of flowcell detectors, under conditions typical of liquid chromatography or flow injection analysis, the linear velocities of the fluids flowing through them are usually too high for non-planar diffusion to be a factor (non-planar diffusion is diffusion toward the electrode from the solution from multiple directions in addition to a normal direction). Only in extreme cases (very low flow rates and extremely small electrode size) will non-planar diffusion affect the measured currents in these flowcell detectors.
Nevertheless, measured current densities of electrode arrays are almost always greater than those obtained for single electrodes of similar active electrode surface area under identical hydrodynamic conditions. This increase in current density is attributed to the re-establishment of the bulk concentration of the electroactive species depleted by electrochemical reaction at an electrode element as the solution containing the species travels across the insulating regions between electrode elements of the array. Moldoveanu, S., Anderson, J. L., Numerical Simulation of Convective Diffusion at a Microarray Channel Electrode, 185 J. Electroanal. Chem. 239 (1985); Anderson, J. L., Ou, T. S., Moldoveanu, S., Hydrodynamic Voltammetry at an Interdigitated Electrode Array in a Flow Channel, Part 1. Numerical Simulation, 196 J. Electroanal. Chem. 213 (1985); Cope, D. S., Tallman, D. E., Calculation of Convective-Diffusion Current at Multiple Strip Electrodes in a Rectangular Flow Channel, Implications for Electrochemical Detection, 205 J. Electroanal. 101 (1986). Consequently, each electrode element of the array "sees" bulk or near bulk concentrations. This situation contrasts greatly with the concentration depletion of electroactive material across the surface of a large electrode by electrochemical reaction. Maintenance of bulk density enhances diffusion to the electrode and thus enhances current density. Enhanced current density is desirable because it usually also provides enhanced signal to noise ratios.
Electrode arrays have been fabricated in a number of geometries. The simplest geometry from the standpoint of fabrication is random. Random arrays can be made by combining powders or chips of electrode material with an insulator (e.g., plastic). Weisshaar, D. E., Tallman, D. E., Anderson, J. L., 53 Anal. Chem. 1809 (1981); Falat, L., Cheng, H. Y., 54 Anal. Chem. 2109 (1982). Another type of random array is made using a reticulated electrode material (e.g., reticulated vitreous carbon) with an insulator to fill the pores. Although difficult to characterize geometrically, random arrays are fairly simple to construct from readily available materials. Wang, J., Freiha, B. A., 298 J. Chromatogr. 79 (1984). Arrays of electrodes based on disks have also been fabricated. For example, these arrays have been constructed by sandwiching carbon fibers between glass microscope slides and sealing them in epoxy. Caudill, W. L., Howell, J. O., Wightman, R. M., 54 Anal. Chem. 2531 (1982).
The most popular type of array geometry is the linear array. This electrode geometry is the most efficient in a flowcell when the elements of the electrode array are oriented opposed to the direction of flow, allowing one to obtain the highest current densities and net currents in the space allowed by flowcell dimensions. The most common method for fabricating linear arrays is by thin film technology and lithography. Usually gold is vapor deposited onto a substrate. Bond, A. M., Henderson, T. L. E., Thorman, W., 90 J. Phys. Chem. 2911 (1986); Thorman, W., van den Bosch, P., Bond, A. M., 57 Anal. Chem. 2764 (1985); Fosdick, L. E., Anderson, J. L. 58 Anal. Chem. 2481 (1986); Fosdick, L. E., Anderson, J. L., Baginski, T. A., Jaeger, R. C., 58 Anal. Chem. 2750 (1986). Another method involves etching a pattern in an insulating substrate and filling in the resulting grooves with a gold filler. DeAbreu, M., Purdy, W. C., 59 Anal. Chem. 204 (1987). The major drawbacks of lithographically fabricated arrays based on vapor deposition arise from their three-dimensional nature. The gold lines are on top of the substrate and are on the order of 300-600 nm thick. Apparently no turbulence is caused by these lines in flowcells. However, the layers tend to separate under voltammetric conditions in solution and are mechanically fragile. Thus their utility as something other than laboratory curiosities is limited, because they cannot be subjected to mechanical polishing, a routine procedure for the maintenance of electrodes.
Carbon electrodes, and especially glassy carbon electrodes, are probably the most common type in use today. "Glassy carbon", as used herein, means a relatively hard integral carbon having the ability to be polished to a glassy appearance and having good electrical conductivity. Glassy carbon is particularly described in Carbon: Electrochemical and Physico-Chemical Properties, K. Kinoshita, Wiley and Sons (1988). Glassy carbon electrodes are, in fact, the most used electrode type used in flowcells. They are used routinely "as is" or as the substrate for surface-modified and mercury-coated electrodes. Nevertheless, carbon linear arrays are not known to exist in the prior art. Glassy carbon is a very hard, brittle material that shatters quite easily. Though the material is available in a number of shapes and sizes, including rods, plates, disks, tubes, cones and crucibles, the only geometry that has had any extensive use as an electrode material has been the rod. Rods of glassy carbon, typically 3 mm in diameter, are readily sealed in Teflon.RTM., Kel-F, or some other insulating material to yield a disk electrode of 3 mm diameter. The present invention utilizes machining techniques to produce improved electrode geometries other than a simple disk.
Heretofore a longfelt need has existed for an improved electrode for use in a flowcell detector providing enhanced current density and enhanced signal to noise ratios, and for a method for manufacturing the same.